This invention relates to a signal amplifier circuit and more particularly to a signal amplifier circuit using a field effect transistor having current unsaturated triode vacuum tube characteristics.
Recently, field effect transistor having current unsaturated characteristics like those of a triode vacuum tube disclosed in U.S. Pat. No. 3,828,230 issued to Nishizawa et al on Aug. 6, 1974 (the corresponding Japanese Patent Application was laid open on Mar. 30, 1973) has attracted attention as an active semiconductor amplifying element for an audio signal amplifier circuit.
A field effect transistor and bipolar transistor hitherto used as an active amplifying element have current saturated characteristics like those of a pentode vacuum tube. The aforesaid new field effect transistor excels over these conventional amplifying elements in respect of output power, linearity and frequency characteristics.
The fact that an amplifying element has triode vacuum tube characteristics means that the operating current of the amplifying element varies in response not only to an input signal voltage but also to variation in direct-current supply voltage. With a signal amplifier circuit using an amplifying element displaying triode vacuum tube characteristics, therefore, a varying component in the supply voltage, for example, a hum component is superposed on an output signal voltage. Further, where a direct-coupled signal amplifier circuit includes a field effect transistor having triode vacuum tube characteristics, then changes occur in the drain current of the field effect transistor due to an alternating-current signal being superposed on direct current supply voltage, failing to carry out a desired voltage amplification.